In the Musée des Arts et Métiers in Paris there are some extraordinary machines from across the ages of scientific invention. Here hangs Foucault’s famous Pendulum, whose gentle swings disclose the rotation of the earth, and here also sits another of his great inventions: a multiple contraption comprising a large wooden table under which rests an enormous pair of bellows. On top of the table there are various clamps and tubes of brass construction. There is a tuning fork, a series of seven small round mirrors on spindles, and a small lens able to spin freely and at great speed. There is something a bee-keeper would recognize as a smoke generator. With this unlikely assortment of instruments, in 1862 Foucault determined the speed of light to within 0.6 per cent of its currently accepted value. Next to the machine is a larger but at first glance similar-looking apparatus. On the whole it seems a clumsy, unsophisticated ancestor of Foucault’s cunning device, waiting in vain to evolve into something that actually works; but in fact its true function remains a mystery to this day.
